The present invention relates to improvements in reflectors and in particular to high reflectance (above 92%) reflectors formed onto a substrate by the vacuum deposition of various materials to form a laminate that has a reflectivity that is greater than the reflectivity of the substrate alone. The invention is particularly suited, and will be described with reference to, the deposition of a high reflective surface onto an aluminum web.
The present invention relates to high reflectance reflectors, and as such, presents the challenge of producing a reflector that can reflect 92% or more of incident light while being durable enough to function in commercial settings. To prevent scratching and dulling from cleaning or handling, many reflector designs dispose clear protective overcoats to the reflecting surface. Such designs, however, sacrifice reflectivity for durability, as any such protective overcoat will be a source of light absorption and/or interfere with the optical design, all with the result of reducing the overall reflectivity. The present invention relates to those reflectors that strive for reflectivities that are incompatible with the use of protective overcoats. By the present invention, both durability and high reflectivity are achieved without the use of protective overcoats.
The invention will be described primarily with regard to aluminum substrates (webs), since they enjoy many commercial advantages over other materials. The invention is not so limited, however, as the unexpected result of durability results from a thick vacuum deposited oxide layer on a smooth substrate and disposed under a vacuum deposited aluminum reflectance layer. The base material, which must be smooth, can be any material, such as steel, that can be made very smooth and subjected to the vacuum deposition process. Other materials are capable of meeting that criteria as well.
The basic method for the vacuum deposition of a high reflective surface onto an aluminum web has been practiced for many years and is thus well known in the art. This well known method has been practiced on webbing in the form of individual sheets of glass or aluminum, typically 40 inches wide, 0.020 inches thick and 50 inches long, weighing approximately four pounds. In the present invention, a coil of aluminum, typically 40 inches wide, 0.020 inches thick and from 1000 to 1500 feet long, weighing 900 to 1400 pounds or more is unwound from an unwinding coil onto a winding coil and the high reflective coatings are vacuum deposited onto the traveling web in stages as the web travels through various vacuum chambers between the unwinding coil to the winding coil.
This method is effective to transform a web, such as polished anodized aluminum (called lighting sheet) having a reflectivity of about 85% into lighting sheet having a reflectivity of about 95% or more. This increase in reflectivity is significant when, for example, the sheet is used for reflectors for increasing the light output from lighting fixtures.
One basic, known method of transforming lighting sheet into reflectors includes the following steps. A sheet of anodized aluminum is inserted into a vacuum chamber which is subdivided into a series of internal vacuum compartments which are separated by seals that permit each compartment to be at the particular pressure suitable for the process being performed within that compartment. The anodized aluminum sheet is transported from one compartment to another to permit whatever process is being performed in that compartment to be applied to the sheet. To fully appreciate the present invention, it is necessary to understand the structure of the surface coating on anodized aluminum.
The Surface Treatment and Finishing of Aluminum and Its Alloys. (Fifth Edition, 1987, pp. 324-368) describes the basic reaction in all anodizing processes as the conversion of the aluminum surface to aluminum oxide while the part is made the anode in an electrolytic cell. The anodic process creates a film having a porous coating having a hexagonal cell structure (pp. 324). Since the reasons for anodizing are inter alia, to permit subsequent plating, to permit application of photographic and lithographic emulsion, etc., the inherent porosity of the anodic film enhances the electroplating and offers a mechanical means of holding an emulsion.
For lighting sheet, however, the porous surface of the anodized aluminum web is not desired and requires that the material be heated to drive off the water (to prevent corrosion by the trapped water attacking the underlying aluminum substrate) and sealed (to prevent the collection of additional dust and water at the surface). Because the vacuum deposition of an aluminum layer directly onto anodized aluminum has been observed to result in corrosion, we have developed the practice of vacuum depositing a thin layer of oxide (such as SiO.sub.2) over the anodized surface to prevent this corrosion from occurring.
Thus, in the first compartment, the aluminum sheet is exposed to heat to drive out water.
In the next compartment, the sheet is exposed to a glow discharge, typically maintained by argon gas, to drive out any remaining water and for further heating to facilitate adhesion.
Next, the sheet is placed into a compartment where a thin oxide layer (typically 0.001 microns) is applied to increase adhesion and prevent the anodized (porous aluminum dioxide) layer which covers the polished aluminum sheet stock from causing corrosion.
In the next compartment, an opaque layer of pure aluminum is deposited onto the web. This layer, not the substrate base, becomes the bottom reflector for incident light. The thickness of this layer is nominally 600 .ANG., although the thickness is not critical. At the lower end of the thickness range, the layer must be thick enough to be opaque and at the upper end, not so thick as to lose its optical qualities.
In the following compartments, the classical reflectance-enhancing layers of a quarter wave length of low refractive index material, then a quarter wave length of a high refractive index material are deposited onto the vacuum deposited aluminum layer. It will be understood by those skilled in the art that "low" and "high" as used in connection with the refractive index of reflectance-enhancing layers are relative terms. The low index materials are lower than the high index materials. The absolute values are less important, although well known in the art. Similarly, the "quarter wave length" thicknesses will be understood to be of a quarter wave length of nominally 550 nanometers (center of visible spectrum). See U.S. Pat. No. 5,007,710 to Nakajima et al.
The high index of refraction material is of the kind deposited in an atmosphere of a partial pressure of oxygen and a partial pressure of water vapor. Each coating is applied in a vacuum compartment to which the sheet is transported after the preceding coating has been applied.
The present invention teaches the use of un-anodized or very lightly anodized aluminum web material in place of standard, full anodized aluminum to reduce cost without sacrificing performance. The un-anodized aluminum is electro-polished, roll polished, mill finished, embossed or subjected to any other process or treatment that gives the required surface smoothness. It is to be understood that wherever the term "polished" is used in the following description and claims it is to include any high reflectance finish, including without limitation, finishes achieved by electro-polishing and roll polishing.
An important aspect of the invention is found in the aluminum base material used as the substrate. Unlike the prior art that teaches an aluminum base that has been anodized (lighting sheet) to provide the necessary hardness and strength, the present invention utilizes a base material of un-anodized aluminum or very lightly anodized aluminum to which is then applied a very thick layer of oxide (such as SiO.sub.2). This oxide layer is thick relative to oxide layers applied over anodized aluminum, but is thinner than the typical anodized layer (1.7 microns) regularly found on lighting sheet. Thus, where we have previously used a thin oxide layer (less than 0.001 microns) over a thick anodized oxide layer to prevent corrosion which the anodized layer tends to promote, the present invention uses a thick oxide layer, orders of magnitude thicker than the corrosion resistant layer taught by the prior art, but thinner than the standard anodized layer.
By applying the relatively thick oxide layer onto the un-anodized or lightly anodized aluminum substrate by vacuum deposition, the structure gains the advantage of a virtually non-porus surface. As reported in Thin Solid Films, International Journal on the Science and Technology of Thin and Thick Films, Vol. 156 No. 1, Jan. 15, 1988:
Thin layers of SiO.sub.2 formed by vacuum deposition are non-porus and electron micrographs show they have smooth, glass-like surfaces. These non-porous films prevent any water and dust particles from penetrating underlying materials.
For aluminum based reflectors, the substitution of a single vacuum deposited layer of an oxide such as SiO.sub.2 for an anodized layer and a corrosion resistant layer of oxide results in a reflector both less expensive to produce and unexpectedly more durable. This unexpected durability is a product of not following our prior technique of depositing a thin oxide layer (over the anodized layer), but rather applying a relatively thick vacuum deposited layer (greater than 0.5 microns). Because an anodized oxide layer is cellular in structure (spongy) above approximately 0.35 microns thick, the elimination of the anodized layer altogether or the use of only a thin anodized layer and the substitution in place of a standard anodized layer with a solid, glass-like material results in a far superior structure.
It is an object of the invention to provide a high reflectance reflector having a laminated structure bonded to a substrate wherein the structure includes a layer of oxide greater than 0.5 microns thick as a base for a layer of pure aluminum to which a quarter wave length thick layer of a low index of refraction material and a quarter wave length thick layer of a high index of refraction material are applied.
Another object of the invention is to apply such a structure to a substrate of a lightly anodized, polished aluminum.
Other objects of the present invention will in part be obvious and will in part appear hereafter.